A number of different methods have been employed for wastewater treatment. These methods involve biochemical oxidation by bacteria, i.e., biomass, to convert organic pollutants to additional biomass, carbon dioxide and water. A common example is the activated sludge process which utilizes an aeration tank or vessel, a setting tank or clarifier, and a pump or pumps to recycle biomass. Wastewater is mixed with active biomass in the aeration tank where biochemical oxidation takes place. The effluent from the aeration tank is then delivered to the settling tank where the biomass settles and some of it is returned to the aeration tank. Treated water from the settling tank exits generally at the top of the tank and is released for discharge or subsequent treatment.
In order for the biochemical oxidation reaction to take place without odor, oxygen is supplied to the mixture in the aeration tank. The demand for oxygen, i.e., oxygen transfer, is dependent upon the amount of biodegradable pollutants available to the biomass or bacteria. Oxygen is made available to the biomass in the form of dissolved oxygen by dissolution of oxygen into the mixture from the aerating gas. Air is the most common source of gas for dissolution of oxygen into the mixture when using open-top aeration tanks. High-purity oxygen can also be used as the aerating gas, but the aeration tank or basin must usually enclose the gas space above the mixture in order to achieve a high oxygen utilization. Venting of the aeration tank or basin is necessary to prevent the buildup of carbon dioxide produced in the bio-oxidation process occurring in the enclosed aeration tank.
In an oxygen-enriched, activated sludge process, the aeration tank is usually separated into two or more closed-tank chambers which operate in series. Wastewater and sludge returned from the settling tank are input to the first chamber and the mixed liquor flows through subsequent chambers in the series and eventually to the settling tank or clarifier. High-purity oxygen gas containing from 60 to 99% oxygen by volume is normally input to the gas space of the first chamber or to the recirculating gas going to the first chamber compressor. The headspace gas flows through the gas space of subsequent chambers concurrent with the mixture. Some versions of the process allow wastewater, return sludge, and high-purity oxygen to be input to any or all chambers of the reactor. High-purity oxygen is usually supplied to the oxygen activated sludge process from an on-site oxygen generating plant or it may be supplied directly from a commercial pipeline.
Various types of mixing devices are used to enhance oxygen transfer to the mixture of each chamber where dissolved oxygen is consumed by the biological reactions. For example, U.S. Pat. No. 4,163,712 describes the use of pressurized water that has been oxygen-enriched as an aeration source by introducing the oxygenated water into the bottom of an aeration tank via a sparger. The oxygenated water is supplied at high velocity so that the water acts as “jets” to provide mixing within the aeration tank while also providing a source of oxygen. U.S. Pat. No. 4,341,633 describes the use of surface aerators and submerged mixers for providing enriched oxygen gas to an enclosed aeration tank. The use of the submerged mixers is described as being necessary for the distribution of the gas throughout the tank. U.S. Pat. No. 4,430,224 describes the use of oxygen-rich gas with enclosed aeration tanks. Oxygen-enriched gas is circulated to a gas distributor located at the bottom of the aeration tank. Mixers are also provided for distributing the introduced gas within the mixture. The above-described systems are typically suitable for shallow aeration tanks, e.g., tanks with a liquid depth of about 15 feet or less.
U.S. Pat. No. 5,240,600 describes the aeration of enclosed, cylindrical bioreactors, which are described as having a depth from 5 to 200 feet. Air is introduced at the bottom of the bioreactor through a sparger to deliver coarse bubbles, i.e., bubble diameter greater than 80 microns, and a biological contactor for further holding the microorganisms within the mixture contained in the deep bioreactor. Additionally, the wastewater is described as being further treated with fine gas bubbles with a bubble diameter much smaller than 80 microns delivered under high velocity to further provide oxygen enrichment. The described system of the '60 Patent is quite complicated with numerous equipment and recycles, making the process expensive and difficult to operate.
Accordingly, there is a need for economical aeration means for deep tanks, such as tanks with liquid over twenty feet in depth without the complication of added equipment to ensure adequate mixing and oxygen supply to microorganism within the aeration tank.